Radiation-sensitive compositions are routinely used in the preparation of imageable materials including lithographic printing plate precursors. Such compositions generally include a radiation-sensitive component, an initiator system, and a binder, each of which has been the focus of research to provide various improvements in physical properties and thereby imaging performance.
Recent developments in the field of printing plate precursors concern the use of radiation-sensitive compositions that can be imaged by means of lasers or laser diodes, and more particularly, that can be imaged and/or developed on-press. Laser exposure does not require conventional silver halide graphic arts films as intermediate information carriers (or “masks”) since the lasers can be controlled directly by computers. High-performance lasers or laser-diodes that are used in commercially-available image-setters generally emit radiation having a wavelength of at least 700 nm, and thus the radiation-sensitive compositions are required to be sensitive in the near-infrared or infrared region of the electromagnetic spectrum. However, other useful radiation-sensitive compositions are designed for imaging with ultraviolet or visible radiation.
There are two possible ways of using radiation-sensitive compositions for the preparation of printing plates. For negative-working printing plates, exposed regions in the radiation-sensitive compositions are hardened and unexposed regions are washed off during development. For positive-working printing plates, the exposed regions are dissolved in a developer and the unexposed regions become an image.
Radiation-sensitive compositions and imageable elements have also been designed for imagewise exposure and/or development while on a printing press using fountain solutions and/or printing inks, as described for example in U.S. Patent Application Publication 2004/0260050 (Munnelly et al.) and WO 2004/101280 (Munnelly et al.). These properties reduce the steps (for example, separate development) required for imaging and printing.
Radically polymerizable compositions that can be used in imageable elements are also described, for example, in U.S. Pat. No. 4,228,232 (Rousseau). Such compositions can be prepared using urethane oligomers that can comprise ethylenically unsaturated polymerizable groups as well as carboxy groups.
Problem to be Solved
Negative-working, on-press developable imageable elements are known, for example, from U.S. Pat. No. 6,582,882 (Pappas et al.) and U.S. Pat. No. 6,899,994 (Huang et al.) in which the imaging composition contains a polymeric binder comprising polyethylene oxide segments. Such polymeric binders have been found to enhance differentiation between exposed and unexposed image areas by facilitating the developability of the unexposed areas and the durability of the exposed image areas.
Such imaging compositions are also known to include radically polymerizable components that include urethane acrylates that polymerize upon thermal imaging. Such polymerizable components have been found to provide desired photospeed, run length, development, and solvent resistance, especially for on-press development.
However, we have found that the resulting dried coating containing a polymerized urethane acrylate may have undesirable tackiness that reduces manufacturing yield. Efforts to minimize this problem include reducing the coating thickness, but this leads to other problems and reduces overall printing plate performance including reduced photospeed and run length and weaker print out.
Thus, there is a need for improved radiation-sensitive compositions that when coated on suitable substrates, provide less tacky coatings without a loss in other desirable properties for imaging and development in printing plate precursors.